In recent years, an apparatus based on a method such as Moving Picture Experts Group (MPEG) in which image information is treated as digital data, and at that time, the image information is compressed by orthogonal transform such as discrete cosine transform, and motion compensation by using redundancy unique to the image information for transmission and accumulation of information with high efficiency, is widely used for both information distribution in a broadcasting station or the like, and information reception at home.
In particular, MPEG2 (International Organization for Standardization (ISO)/International Electrotechnical Commission (IEC) 13818-2) is defined as a general-purpose image encoding method, and is currently widely used in a broad range of application for professional use and consumer use, as standards covering all of an interlace scanning image and a sequential scanning image, a standard resolution image, and a high definition image. By using the MPEG2 compression method, a high compression ratio and excellent image quality can be realized by assigning a coding rate (bit rate) of 4 Mbps to 8 Mbps in a case of a standard-resolution interlace scanning image including 720×480 pixels, and a coding rate of 18 Mbps to 22 Mbps in a case of a high-resolution interlace scanning image including 1920×1088 pixels.
MPEG2 was mainly used for high image quality encoding suitably used for broadcasting, but was not compatible with an encoding method using a lower coding rate (bit rate) than that of MPEG1, that is, at a higher compression rate. Needs for such an encoding method was expected to be increased in the future with spread of portable terminals, and standardization of a MPEG4 encoding method was performed in response thereto. For standards of an image encoding method, standards of ISO/IEC 14496-2 were approved as international standards in December, 1998.
In addition, in recent years, first, standardization of H.26L (International Telecommunication Union Telecommunication Standardization Sector (ITU-T) Q6/16 Video Coding Expert Group (VCEG)) is in progress with the aim of image encoding for teleconferencing. H.26L requires a larger calculation amount due to encoding and decoding, compared to the encoding method of the related art such as MPEG2 or MPEG4, but is a well-known method for realizing higher encoding efficiency. In addition, currently, as the part of an operation of MPEG4, standardization for realizing higher encoding efficiency by even introducing functions not supported in H.26L, based on H.26L, is performed as Joint Model of Enhanced-Compression Video Coding.
In a schedule of the standardization, H.264/MPEG-4 Part 10 (Advanced Video Coding, hereinafter, referred to as AVC) became international standard in March, 2003.
However, there was a concern that a macro block size of 16 pixels×16 pixels may not be optimal with respect to a large frame of Ultra High Definition (UHD; 4000 pixels×2000 pixels), which may be a target of a next-generation encoding method.
Herein, currently, for further improvement of the encoding efficiency by AVC, standardization of an encoding method called High Efficiency Video Coding (HEVC) is in progress by Joint Collaboration Team—Video Coding (JCTVC) which is a joint standardization organization of ITU-T and ISO/IEC (for example, see NPL 1).
In the encoding method of HEVC, a coding unit (CU) is defined to be the same processing unit as the macro block of AVC. Unlike the macro block of AVC, the size of the CU is not fixed to 16×16 pixels, and is designated in image compression information in respective sequences.
However, in order to improve encoding of a motion vector using median prediction of AVC, adaptively using any one of “temporal predictor” and “spatio-temporal predictor” as predictive motion vector information, in addition to “spatial predictor” which is defined in AVC and is acquired by median prediction (for example, see NPL 2), has been proposed.
In an image information encoding device, a cost function in a case of using each predictive motion vector information item, is calculated for each block, and selection of optimal predictive motion vector information is performed. In the image compression information, graph information showing information indicating which predictive motion vector information is used with respect to each block is transmitted.
In addition, as one of encoding methods of motion information, a method called Motion Partition Merging (hereinafter, also referred to as a merge mode) is proposed (for example, see NPL 3). In this method, in a case where motion information of a corresponding block is the same as motion information of peripheral blocks, only flag information is transmitted, and the motion information of the corresponding block is reconstructed using the motion information of the peripheral blocks when performing decoding.